In the electrodeposition of metals or alloys, in particular of zinc or tin or zinc alloys or tin alloys, onto metallic substrates, such as shaped metal articles or metallated plastic shaped articles, from aqueous acidic solution, a bright coating is frequently desirable in order to give the electroplated article an advantageous decorative appearance. Thus, in addition to the corrosion-inhibiting effect or an effect which improves the mechanical properties of the shaped article, a decorative effect is often desirable. In order to obtain the desired effects, it is essential that the electroplating baths contain certain assistants, since otherwise the metal coatings being deposited from acidic solution are generally dull and frequently also irregular. A group of such assistants for acidic electroplating baths comprises, for example, conductive salts which are used for improving the conductivity of the baths. Another group of assists comprises the brighteners.
U.S. Pat. No. 3,694,330 (Reissue 27 999) discloses acidic electroplating zinc baths which contain ammonium salts and, as brighteners, aromatic carbonyl compounds. Aromatic carboxylic acids and aromatic aldehydes and ketones are mentioned as the latter. Explicitly mentioned are, inter alia, cinnamic acid, cinnamaldehyde, benzoic acid, benzalacetone and ethyl benzoylacetate.
U.S. Pat. No. 4,222,908 describes acidic electroplating zinc baths which contain sulfamate ions and, as brighteners, aromatic carbonyl compounds. Aromatic aldehydes and ketones are mentioned, including benzalacetone as a preferred substance.
DE-A-29 48 261 describes acidic electroplating zinc baths which may contain ammonium salts and contain, as brighteners, aromatic, carbonyl-containing compounds, such as o-chlorobenzaldehyde, p-chlorobenzaldehyde, o-hydroxybenzaldehyde, aminobenzaldehyde, veratraldehyde, benzylideneacetone (benzalacetone), coumarin, 3,4,5,6-tetrahydrobenzaldehyde, acetophenone, propiophenone, furfurylideneacetone, 3-methoxybenzalacetone, benzaldehyde, vanillin, hydoxybenzaldehyde, anisaldehyde, benzoic acid, sodium benzoate, sodium salicylate or 3-pyridinecarboxylic acid (nicotinic acid). The use of polymeric sulfur-containing compounds of a defined general formula is described here for improving the properties of the electroplating baths.
The most important member of such brighteners for acidic zinc baths is benzylideneacetone (benzalacetone). Owing to its marked vapor pressure, benzylideneacetone can evaporate from electroplating baths, leading to losses of substance. Furthermore, these compounds and their homologs cause allergic reactions in many people who handle them, such as skin disorders and itching. In addition, benzalacetone is not water-soluble and must be solubilized with the aid of, in general, toxic solvents. These solvents frequently have a low flashpoint, necessitating additional safety measures.
DE-A-41 19 341 (2) recommends replacing brighteners based on benzylideneacetone in aqueous acidic electroplating baths for the deposition of zinc or zinc alloys by certain other substances which achieve the efficiency of the known brighteners but do not adversely affect the health of persons who handle these substances. However, the compounds disclosed there have the other disadvantages mentioned above.
It is an object of the present invention to provide compounds as brighteners in aqueous acidic electroplating baths, which compounds achieve the efficiency of the known brighteners, in particular of benzylideneacetone, and have a lower vapor pressure as a single substance and in the electroplating baths, in order to avoid losses of substance. The novel compounds should furthermore be water-soluble so that organic solvent can be dispensed with and the concentration of solubilizers in the elecroplating baths can be reduced.